An internal combustion engine, in particular, an automotive engine, has intake and exhaust valves for intake of air-fuel mixture and exhaust. At the time of intake and exhaust, the intake and exhaust valves are opened and closed. A cam shaft controls the opening and closing of the intake and exhaust valves.
Types of operation of opening the intake and exhaust valves by cams of the cam shaft are classified into a direct acting type in which cams directly press valve lifters for opening and closing valves, and a rocker arm type in which cams press valves via arms called rocker arms for opening and closing valves.
The intake and exhaust valves thermally expand because of heat from combustion chambers. When the valves thermally expand, intake and exhaust timings change accordingly. Thus, in order to absorb such a change caused by thermal expansion, in a condition in which a cam does not press a valve lifter or a cam shaft abutment portion (a roller or a slipper surface) of a rocker arm, a clearance (valve clearance) is provided for adjustment between the cam and the valve lifter (or the cam shaft abutment portion of the rocker arm).
In assembly of an engine, usually, after assembly of the valves to a cylinder head, corresponding valve clearances are automatically adjusted by a valve clearance adjusting apparatus. Subsequent to the adjustment, the valve clearances are actually measured for confirmation.
An example of a valve clearance adjusting method is disclosed in, for example, Patent Document 1. In this valve clearance adjustment method, an adjust screw of a rocker arm is rotated in the normal direction so as to press a valve by a rocker arm by a displacement amount greater than a valve clearance. After that, the adjust screw is rotated in the reverse direction to as to return the valve to an ordinal home position. A point in time at which the valve stops its displacement after having returned to the home position is detected by displacement stoppage detection means, and the position of the rocker arm at that time is recognized as a zero point position at which the clearance is zero. After that, the adjust screw is rotated in the same reverse direction by a predetermined angle so as to set a valve clearance, and then locked by a lock nut such that the adjust screw becomes non-rotatable. The term “valve clearance” used herein refers to a clearance between a valve-side end portion of a rocker arm and a top portion of a bridge member which connects valve retainers of left and right valves.
An example of a valve clearance adjusting device is disclosed in, for example, Patent Document 2. This valve clearance adjusting device is configured to set a valve clearance between an adjust screw and a valve in an actual machine in which a cylinder head and a cylinder block have been assembled. An adjustment head for adjusting valve clearance is provided to be movable in the vertical direction along guide rails, and is moved up and down over a predetermined stroke by means of an elevating actuator such as a cylinder.
Furthermore, a roller rocker arm (rocker arm of a fulcrum adjustment type) which does not include a rocker shaft is described in Patent Document 3.